In its current incarnation, we've got a new front-end for MXR. In subsequent releases we'll be adding the documentation and other sections.

As you use it, please note:

This is MXR, and MXR has lots of hairy bits. I'm not (yet) making changes to how it indexes and such. If you find those sorts of bugs, report them in Webtools/LXR.

I'm not regularly updating the source/index -- it's a static snapshot of Firefox trunk from last week.

The UI is not fixed in stone. In fact, I hope you'll give lots of constructive suggestions about how to make this even better.
I'm actively working on this, so if it breaks, I apologize. I wouldn't recommend you rely on this yet.